1. Field of the Invention
The present invention relates to a gas fumigation method and system. More particularly, the present invention relates a method and system for fumigating a region with a non-flammable gaseous mixture containing phosphine.
2. Description of Related Art
In agricultural and food processing industries, infestation by pests, such as rodents and insects, can cause significant problems. Pests can contaminate stored crops or food products rendering them unfit or unappealing for use. Pests consuming products not only cause a loss of inventory, but also may damage products and thereby reduce the product value.
Current means of controlling pests involve the use of fumigants and product protectants. Fumigants are used to eliminate pests inhabiting a product, and protectants are used on pest free products to reduce the risk of infestation.
One type of fumigant is the chemical phosphine (PH.sub.3). Phosphine is a poisonous and highly flammable gas capable of self ignition under certain atmospheric conditions. Phosphine gas is advantageous because it does not leave a significant residue on a product after being used as a fumigant.
Phosphine gas is typically generated using a metal phosphide, such as aluminum phosphide or magnesium phosphide, often in the form of pellets or tablets optionally placed in gas permeable packets. When exposed to moisture, typically ambient humidity, phosphine gas is generated. The generation of phosphine gas is dependent not only on the available moisture, but on temperature and time as well. Because metal phosphide does not react as rapidly at lower temperatures, this material is not generally used at temperatures below 40 degrees Fahrenheit. The time required to generate the available phosphine gas can range from 2 to 8 days depending on the temperature. The control of phosphine concentrations resulting from the reaction is difficult, and a wide range of concentrations are produced.
A common fumigation process using metal phosphides involves placing the material in contact with stored product to be treated, such as on top of the product or buried within the product. A number of methods can be used to facilitate distribution of phosphine gas throughout the product and supply the necessary moisture for supporting the reaction.
The use of metal phosphides creates a hazardous environment for individuals conducting fumigation. As soon as these materials are exposed to atmospheric moisture, the reaction begins to release flammable and toxic phosphine gas. Because workers can be immediately exposed to the gas, protective clothing and breathing equipment are necessary. In addition, individuals often must work in a confined space, and multiple workers are required to expedite the process and to provide rescue if needed.
Once started, the generation of phosphine gas will continue until either all of the metal phosphide has reacted, or the moisture supporting the reaction is eliminated. Eliminating the moisture would, in theory, require removal of the materials from an area being fumigated and placement in a suitable sealed container. This container would have to be capable of accepting the phosphine gas still being generated as the available moisture is used. This is not done in practice because of the high potential for explosions resulting from self igniting phosphine concentrations.
Once the fumigation is complete, the residual material from the metal phosphide reaction must be removed from the fumigated area. These remaining materials must be treated to ensure that all of the available phosphine gas has been generated, a process which can still result in flammable and toxic vapors. The residues from the treatment must be packaged and prepared for proper disposal.
Use of metal phosphides is not limited to their direct placement in contact with a stored product. Means to react the material outside of a storage region have been developed and used, however, special care must be taken to limit the phosphine concentrations to prevent flammable or self igniting mixtures in air.
After an area is fumigated to destroy pests, certain measures must often be taken to protect the stored product from reinfestation. The current means for protecting products from reinfestation involve spraying liquid chemicals directly onto the products.
The application of chemical protectants is most effective when done while the stored product is in motion, for example, during the loading of a storage container or vessel. This facilitates the distribution of the protectant throughout the product. Application of the chemical after the product is in storage requires movement of all of the product, for example, to another storage container. This requires additional storage space, and the movement sometimes results in physical damage to the product.
Chemical protectants are often mixed with water or other dilutents to establish a suitable concentration at the facility where they are applied. Steps must be taken to ensure that the diluted chemical remains mixed and that the mixture is used within a short period of time after preparation. Handling of such chemicals is hazardous to humans, and special precautions must be taken to ensure worker safety. These chemicals are also hazardous to the environment, and wastes resulting from their use must be properly disposed.
Phosphine gas also has the potential of being used as a protectant chemical. At sufficient concentrations, phosphine can reduce the chances of reinfestation of a stored product. Introduction of phosphine into a storage can provide desired protection without the need to move the product. Phosphine can be used over a long period of time to provide the desired protection, whereas the effectiveness of liquid chemical protectants decreases to zero with time.
When phosphine is mixed with other gases, the resulting mixture can be effective as a fumigant or protectant, while simultaneously eliminating the flammability hazards. For example, U.S. Pat. No. 4,889,708, which is incorporated by reference in its entirety, discloses a pressurized mixture of a diluent gas and phosphine gas in a container adapted to release the mixture of gases during fumigation. However, there is a need for systems capable of managing the flow of such gaseous mixtures during fumigation of particular areas. In addition, there is a need for systems capable of mixing gases on site during fumigation to allow for separate supply of components of the mixtures.
In light of the foregoing, there is a need in the art for improving fumigations by limiting potentially flammable concentrations of phosphine, reducing worker risk, eliminating the need to retrieve, treat, and dispose of metal phosphide materials, having the ability to stop the introduction of phosphine at will, and producing phosphine gas independently of environmental conditions. Furthermore, there is a need in the art for providing protection to stored products over extended periods of time.